U.S. patent number 6,907,307 [Application Number 10/461,957] was granted by the patent office on 2005-06-14 for support volume calculation for a cad model.
This patent grant is currently assigned to 3D Systems, Inc.. Invention is credited to Yong Chen, Rajeev B. Kulkarni.
United States Patent |
6,907,307 |
Chen , et al. |
June 14, 2005 |
Support volume calculation for a CAD model
Abstract
In solid freeform fabrication processes that make use of a
removable support material, pre-calculation of the amount of
support material needed for a build is difficult (inaccurate or
slow) because the digital data for generating the support material
is often not generated until the build is in progress. A method is
proposed that has been shown to generate rapid and accurate
estimates of the amount of both build and support material needed
before a build begins, to accurately predict before a build begins
when replenishment materials are needed, and to track material
consumptions over time.
Inventors: |
Chen; Yong (Valencia, CA),
Kulkarni; Rajeev B. (Valencia, CA) |
Assignee: |
3D Systems, Inc. (Valencia,
CA)
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Family
ID: |
29999514 |
Appl.
No.: |
10/461,957 |
Filed: |
June 13, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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188574 |
Jul 2, 2002 |
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Current U.S.
Class: |
700/119; 264/401;
427/466; 700/118; 700/163; 700/98; 700/120; 427/470; 264/516;
264/512 |
Current CPC
Class: |
B33Y
30/00 (20141201); G05B 19/4099 (20130101); B29C
64/106 (20170801); B33Y 50/02 (20141201); B33Y
40/00 (20141201); B29C 2035/0283 (20130101); G05B
2219/49013 (20130101); G05B 2219/49038 (20130101); G05B
2219/49016 (20130101) |
Current International
Class: |
G06F
19/00 (20060101); G06F 019/00 () |
Field of
Search: |
;700/118,119,97,98,163
;345/419,420 ;264/75,401,308,633,642 ;427/472,470,466 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
US. Appl. No. 09/970,956 "Quantitized Feed System for Solid
Freeform Fabrication"..
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Primary Examiner: Picard; Leo
Assistant Examiner: Lee; Douglas S.
Attorney, Agent or Firm: Ervin; Michael A. D'Alessandro;
Ralph
Parent Case Text
This application is a continuation-in-part of application Ser. No.
10/188,574 filed Jul. 2, 2002 now abandoned.
Claims
We claim:
1. In a solid freeform fabrication process wherein an object is
formed with a removable support material, the process including the
steps of a) rapidly and accurately predicting a volume and weight
of said support material required to form said object prior to
commencing the building of said object, b) predicting when
replenishment of said support material is required, and c) tracking
consumption of said support material over time.
2. In a solid freeform fabrication process wherein an object is
formed with a removable support material, the process for rapidly
and accurately predicting a volume and weight of said support
material required to form said object prior to commencing the
building of said object, the process comprising a) selecting the
location and orientation of said object in the build chamber, b)
calculating a total sweeping body volume associated with said
object, c) calculating the volume of said object, d) subtracting
said object volume from said total sweeping body volume to give a
sweeping body support material volume, d) multiplying said sweeping
body support material volume by a density of support structure to
give a support material weight, and e) adding an estimated weight
of waste support material.
3. The process of claim 2 wherein the calculation of said sweeping
body volume comprises: a) calculating the x-y extent boundaries of
said object, c) mapping an x-y grid of dimension d within said
boundaries, c) identifying for each element of said grid the
associated z-height of the top most element of the object below the
grid, d) multiplying the grid areas (d.times.d) by the each
associated z-height to calculate the volumes of the rectangular
blocks, and summing the said volumes of rectangular blocks to
calculate the total sweeping volume.
4. The process of claim 3 wherein said dimension d is between 3 and
54 mils.
5. The process of claim 4 wherein said dimension d is between 6 and
26 mils.
6. In a solid freeform fabrication process wherein an object is
formed from a build material with a removable support material, the
process including tho steps of a) rapidly and accurately predicting
a volume and weight of said build material required to form said
object prior to commencing the building of said object, b)
predicting when replenishment of said build material is required,
and c) tracking consumption of said build material over time.
7. The process of claim 6 wherein said step of rapidly and
accurately predicting said volume and weight of said build material
required to form said object prior to commencing the building of
the object comprises a) calculating the build material volume of
the triangulated STL model of said object from the sum of the
signed volume of each tetrahedron formed by a triangle and an
original point, b) multiplying said build material volume by a
density of build material to give a build material weight, and c)
adding an estimated weight of waste build material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to solid freeform fabrication, and
in particular to those solid freeform fabrication techniques that
build objects in a layer-wise fashion and require a support
structure for the build. This can include stereolithography,
selective deposition modeling, and direct composite manufacturing
using pastes or semi-solid materials.
2. Description of the Prior Art
Several technologies have been developed for the rapid creation of
models, prototypes, and parts for limited run manufacturing. These
technologies are generally called Solid Freeform Fabrication
techniques, and are herein referred to as "SFF". Some SFF
techniques include stereolithography, selective deposition
modeling, laminated object manufacturing, selective phase area
deposition, multi-phase jet solidification, ballistic particle
manufacturing, fused deposition modeling, particle deposition,
laser sintering, direct composite manufacturing and the like.
Generally in SFF techniques, complex parts are produced from a
modeling material in an additive fashion as opposed to conventional
fabrication techniques, which are generally subtractive in nature.
For example, in most conventional fabrication techniques material
is removed by machining operations or shaped in a die or mold to
near net shape and then trimmed. In contrast, additive fabrication
techniques incrementally add portions of a build material to
targeted locations, layer by layer, in order to build a complex
part. SFF technologies typically utilize a computer graphic
representation of a part and a supply of a building material to
fabricate the part in successive layers, often called laminae.
These laminae are sometimes called object cross-sections, layers of
structure, object layers, layers of the object, or simply layers
(if the context makes it clear that solidified structure of
appropriate shape is being referred to). Each lamina represents a
cross-section of the three-dimensional object. Typically lamina are
formed and adhered to a stack of previously formed and adhered
laminae. In some SFF technologies, techniques have been proposed
which deviate from a strict layer-by-layer build up process wherein
only a portion of an initial lamina is formed and prior to the
formation of the remaining portion(s) of the initial lamina, at
least one subsequent lamina is at least partially formed.
Generally, in most SFF techniques, structures are formed in a
layer-by-layer manner by solidifying or curing successive layers of
a build material. For example, in stereolithography a tightly
focused beam of energy, typically in the ultraviolet radiation
band, is scanned across a layer of a liquid photopolymer resin to
selectively cure the resin to form a structure. In Selective
Deposition Modeling, herein referred to as "SDM" a phase change
build material is jetted or dropped in discrete droplets, or
extruded through a nozzle, to solidify on contact with a build
platform or previous layer of solidified material in order to build
up a three-dimensional object in a layer wise fashion. Other
synonymous names for SDM used in the industry are: solid object
imaging, deposition modeling, multi-jet modeling, three-dimensional
printing, thermal stereolithography, and the like. Direct
composites manufacturing refers to a layer-wise build technology,
which utilizes slurry pastes of metals or ceramics as the build
material.
In one class of SFF techniques, a three-dimensional object is built
up by applying successive layers of unsolidified, flowable material
to a working surface, and then selectively exposing the layers to
synergistic stimulation in desired patterns, causing the layers to
selectively harden into object laminae which adhere to
previously-formed object laminae. In this approach, material is
applied to the working surface both to areas that will not become
part of an object lamina, and to areas that will become part of an
object lamina. Typical of this approach is Stereolithography (SL),
as described in U.S. Pat. No. 4,575,330, to Hull. According to one
embodiment of Stereolithography, the synergistic stimulation is
radiation from an UV laser, and the material is a photopolymer.
Another example of this approach is Selective Laser Sintering
(SLS), as described in U.S. Pat. No. 4,863,538, to Deckard, in
which the synergistic stimulation is IR radiation from a carbon
dioxide laser and the material is a sinterable powder. A third
example is Three-Dimensional Printing (3DP) and Direct Shell
Production Casting (DSPC), as described in U.S. Pat. Nos. 5,340,656
and 5,204,055, to Sachs, et al., in which the synergistic
stimulation is a chemical binder (e.g. an adhesive), and the
material is a powder consisting of particles that bind together
upon selective application of the chemical binder.
In a second class of SFF techniques, an object is formed by
successively cutting object cross-sections having desired shapes
and sizes out of sheets of material to form object lamina.
Typically in practice, the sheets of paper are stacked and adhered
to previously cut sheets prior to their being cut, but cutting
prior to stacking and adhesion is possible. Typical of this
approach is Laminated Object Manufacturing (LOM), as described in
U.S. Pat. No. 4,752,352, to Feygin in which the material is paper,
and the means for cutting the sheets into the desired shapes and
sizes is a carbon dioxide laser. U.S. Pat. No. 5,015,312 to Kinzie
also addresses building object with LOM techniques.
In a third class of SFF techniques, object laminae are formed by
selectively depositing an unsolidified, flowable material onto a
working surface in desired patterns in areas which will become part
of an object laminae. After or during selective deposition, the
selectively deposited material is solidified to form a subsequent
object lamina that is adhered to the previously formed and stacked
object laminae. These steps are then repeated to successively build
up the object lamina-by-lamina. This object formation technique may
be generically called Selective Deposition Modeling (SDM). The main
difference between this approach and the first approach is that the
material is deposited only in those areas that will become part of
an object lamina. Typical of this approach is Fused Deposition
Modeling (FDM), as described in U.S. Pat. Nos. 5,121,329 and
5,340,433, to Crump, in which the material is dispensed in a
flowable state into an environment which is at a temperature below
the flowable temperature of the material, and which then hardens
after being allowed to cool. A second example is the technology
described in U.S. Pat. No. 5,260,009, to Penn. A third example is
Ballistic Particle Manufacturing (BPM), as described in U.S. Pat.
Nos. 4,665,492; 5,134,569; and 5,216,616, to Masters, in which
particles are directed to specific locations to form object
cross-sections. A fourth example is Thermal Stereolithography (TSL)
as described in U.S. Pat. No. 5,141,680, to Almquist et. al.
In SDM, as well as the other SFF approaches, typically accurate
formation and placement of working surfaces are required so that
outward facing cross-sectional regions can be accurately formed and
placed. The first two approaches naturally supply working surfaces
on which subsequent layers of material can be placed and lamina
formed. However, since the third approach, SDM, does not
necessarily supply a working surface, it suffers from a
particularly acute problem of accurately forming and placing
subsequent lamina which contain regions not fully supported by
previously dispensed material such as regions including outward
facing surfaces of the object in the direction of the previously
dispensed material. In the typical building process where
subsequent laminae are placed above previously formed laminae this
is particularly a problem for down-facing surfaces (down-facing
portions of laminae) of the object. This can be understood by
considering that the third approach theoretically only deposits
material in those areas of the working surface which will become
part of the corresponding object lamina. Thus, nothing will be
available to provide a working surface for or to support any
down-facing surfaces appearing on a subsequent cross-section.
Downward facing regions, as well as upward facing and continuing
cross-sectional regions, as related to photo-based
Stereolithography, but as applicable to other SFF technologies
including SDM, are described in detail in U.S. Pat. Nos. 5,345,391,
and 5,321,622, to Hull et. al. and Snead et. al., respectively. The
previous lamina is non-existent in down-facing regions and is thus
unavailable to perform the desired support function. Similarly,
unsolidified material is not available to perform the support
function since, by definition, in the third approach, such material
is typically not deposited in areas which do not become part of an
object cross-section. The problem resulting from this situation may
be referred to as the "lack of working surface" problem. This
problem and alternate approaches to solving it is described in U.S.
Pat. No. 6,270,335 to Leyden et al.
All patents referred to herein above in this section of the
specification are hereby incorporated by reference as if set forth
in full.
In addition to this "lack of working surface" problem, many of the
build processes used in these technologies often result in stresses
that can result in distortions of the object during the build. In
addition complex objects can have significant overhanging features
during the build, requiring an underlying support to prevent
sagging. For all of the aforementioned issues these SFF techniques
often include the simultaneous building of support structures that
may be used for supporting an overhanging feature, for anchoring
the object during the build, or for providing a working surface for
deposition. These support structures may be a different material or
sometimes the same material. This support material is later removed
to generate the final object. An important and unsolved need for
process planning is the ability to accurately and rapidly predict
before a build the amounts of build and support material needed, to
predict when material replenishment is needed, and to track
material usage over time.
It is straightforward to pre-calculate the volume and therefore the
weight of an object to be made if a CAD or STL model is available
of the object. The difficulty comes in calculating the volume and
weight of the support material, which is not in CAD or STL format,
and will only be calculated and generated during the build on a
slice-on-the-fly basis. Thus there is a need for a method for
accurately and quickly pre-calculating the volume and weight
required for support materials in certain solid freeform
fabrication techniques.
BRIEF SUMMARY OF THE INVENTION
The instant invention provides benefits across a number of SFF
technologies. While the description, which follows hereinafter, is
meant to be representative of a number of such applications, it is
not exhaustive. As will be understood, the basic methods and
apparatus taught herein can be readily adapted to many uses. It is
intended that this specification and the claims appended hereto be
accorded a breadth in keeping with the scope and spirit of the
invention being disclosed despite what might appear to be limiting
language imposed by the requirements of referring to the specific
examples disclosed.
It is an aspect of this invention to provide a method to predict
before a build is made in a solid freeform fabrication process in
which an object is formed with a removable support material the
volume and weight of build and support materials that will be
consumed.
It is a further aspect of this invention to provide a method to
predict before a build is made in a solid freeform fabrication
process in which an object is formed with a removable support
material whether enough build and support materials are available
in the system to complete the build.
It is a further aspect of this invention to provide such a method
that is both accurate and rapid.
The invention includes in a solid freeform fabrication process
wherein an object is formed with a removable support material, the
process including the steps of a) rapidly and accurately predicting
the volume and weight of support material required to form the
object prior to commencing the building of the object, b)
predicting when replenishment of the support material is required,
and c) tracking consumption of support material over time.
Further the invention includes in a solid freeform fabrication
process wherein an object is formed with a removable support
material, the process for rapidly and accurately predicting the
volume and weight of support material required to form the object
prior to commencing the building of the object, the process
comprising a) selecting the location and orientation of said object
in the build chamber, b) calculating a total sweeping body volume
associated with said object, c) calculating the volume of said
object, d) subtracting said object volume from said total sweeping
body volume to give a sweeping body support material volume, d)
multiplying said sweeping body support material volume by a density
of support structure to give a support material weight, and e)
adding the estimated weight of waste support material.
The rapid calculation is done by mapping an X-Y grid across the X-Y
extents of the object model, determining the top-most triangle
existing for each cell of the grid, recording the z-height of that
triangle from the model, using that z-height to calculate the
volume of the rectangular block under each cell, and summing those
volumes to get the entire sweeping body volume. The volume of the
object (in CAD or STL) can then be calculated using conventional
equations and subtracted from the sweeping body to give the support
volume. The weight of the support material can then be calculated
from knowing the density of the support material. If the support
structure is not continuous, but instead is a support web for
example, the density can be adjusted to account for that
difference.
Further the invention also includes in a solid freeform fabrication
process wherein an object is formed from a build material with a
removable support material, the process including the steps of a)
rapidly and accurately predicting a volume and weight of said build
material required to form said object prior to commencing the
building of said object, b) predicting when replenishment of said
build material is required, and c) tracking consumption of said
build material over time.
BRIEF DESCRIPTION OF THE DRAWINGS
The aspects, features, and advantages of the present invention will
become apparent upon consideration of the following detailed
disclosure of the invention, especially when it is taken in
conjunction with the accompanying drawings wherein:
FIG. 1 is a diagrammatic side view of a solid deposition modeling
apparatus.
FIG. 2 is a diagrammatic side view of a preferred embodiment of a
solid deposition modeling apparatus.
FIG. 3 is an example of an object and its supports on the platform
of a solid deposition modeling apparatus.
FIG. 4 is an example of two objects and their supports on the
platform of a solid deposition modeling apparatus.
FIG. 5 is an example of a sweeping body and the object
corresponding to FIG. 3.
FIG. 6 is an example of sweeping bodies with the objects
corresponding to FIG. 4.
FIG. 7 is an example illustrating a grid pattern mapped onto an X-Y
extents of an object.
FIG. 8 is an example illustrating the calculation of the Z value of
a grid for a triangle.
FIG. 9 is an illustration of the mapping from (x, y) to (i, j).
To facilitate understanding, identical reference numerals have been
used, where possible, to designate identical elements that are
common in the figures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the present invention is applicable to many SFF techniques
the invention will be described with respect to a SDM technique
utilizing an ink jet print head dispensing a ultraviolet radiation
curable phase change material. However it is to be appreciated that
the present invention can be implemented with any SFF technique
utilizing a wide variety of materials. For example, the curable
phase change material can be cured by exposure to actinic radiation
having wavelengths other than in the ultraviolet band of the
spectrum, or by subjecting the material to thermal heat. Or the
curing may be affected by a selective exposure from a laser beam,
as in stereolithography. Alternately the build material may not be
a curable material but a material which changes phase due to
temperature, being deposited in a molten state, and quickly
hardening due to cooling.
Referring particularly to FIG. 1 there is illustrated generally by
the numeral 10 a SDM apparatus for practicing an embodiment of an
SDM process. The SDM apparatus 10 is shown building a
three-dimensional object 44 on a support structure 46 in a build
environment shown generally by the numeral 12. The object 44 and
support structure 46 are built in a layer-by-layer manner on a
build platform 14 that can be precisely positioned vertically by
any conventional actuation means 16. Directly above and parallel to
the platform 14 is a rail system 18 on which a material dispensing
trolley 20 resides carrying a dispensing device 24. Preferably the
dispensing device 24 is an ink jet print head that dispenses a
build material and support material and is of the piezoelectric
type having a plurality of dispensing orifices. However, other ink
jet print head types could be used, such as an acoustic or
electrostatic type, if desired. A preferred ink jet print head is
the Z850 print head available from Xerox Corporation of
Wilsonville, Oreg. Alternatively a thermal spray nozzle could be
used instead of an ink jet print head, if desired.
The trolley carrying the print head 24 is fed the curable phase
change build material 22 from a remote reservoir 49. The remote
reservoir is provided with heaters 25 to bring and maintain the
curable phase change build material in a flowable state. Likewise,
the trolley carrying the print head 24 is also fed the non-curable
phase change support material from remote reservoir 50 in the
flowable state. In order to dispense the materials, a heating means
is provided to initially heat the materials to the flowable state,
and to maintain the materials in the flowable state along its path
to the print head. The heating means comprises heaters 25 on both
reservoirs 49 and 50, and additional heaters (not shown) on the
umbilcals 52 connecting the reservoirs to the print head 24.
Located on the print head 24 is a plurality of discharge orifices
27 for dispensing both the build material and support material,
although just one is shown in FIG. 1. Each discharge orifice is
dedicated to dispense either the build material or the support
material in a manner that either material can be dispensed to any
desired target location in the build environment.
A reciprocating means is provided for the dispensing device 24
which is reciprocally driven on the rail system 18 along a
horizontal path by a conventional drive means 26 such as an
electric motor. Generally, the trolley carrying the dispensing
device 24 takes multiple passes to dispense one complete layer of
the materials from the discharge orifices 27. In FIG. 1, a portion
of a layer 28 of dispensed build material is shown as the trolley
has just started its pass from left to right. Discreet dispensed
droplets 30 are shown in mid-flight, and the distance between the
discharge orifice and the layer 28 of build material is greatly
exaggerated for ease of illustration. The layer 28 may be all build
material, all support material, or a combination of build and
support material, as needed, in order to form and support the
three-dimensional object.
The build material and support material are dispensed as discrete
droplets 30 in the flowable state, which solidify upon contact with
the layer 28 as a result of a phase change. Alternatively, the
materials may be dispensed in a continuous stream in an SDM system,
if desired. Each layer of the object is divided into a plurality of
pixels on a bit map, in which case a target location is assigned to
the pixel locations of the object for depositing the curable phase
change material 22. Likewise, pixel coordinates located outside of
the object may be targeted for deposition of the non-curable phase
change material 48 to form the supports for the object as needed.
Generally, once the discrete droplets are deposited on all the
targeted pixel locations of the bit map to establish an initial
layer thickness, a solid fill condition is achieved. Preferably the
initial layer thickness established during dispensing is greater
than the final layer thickness such that the solid fill condition
for each layer contains material in excess of that needed for the
layer.
A planarizer 32 is drawn across the layer to smooth the layer and
normalize the layer to establish the final layer thickness. The
planarizer 32 is used to normalize the layers as needed in order to
eliminate the accumulated effects of drop volume variation, thermal
distortion, and the like, which occur during the build process. It
is the function of the planarizer to melt, transfer, and remove
portions of the dispensed layer of build material in order to
smooth it out and set a desired thickness for the last formed layer
prior to curing the material. This ensures a uniform surface
topography and layer thickness for all the layers that form the
three-dimensional object, however it produces waste material that
must be removed from the system. The planarizer 32 may be mounted
to the material dispensing trolley 20 if desired, or mounted
separately on the rail system 18, as shown.
The planarizer 32 is utilized in SDM building techniques that
deposit build material in excess of a desired thickness for each
layer according to data of a prescribed pattern for each layer, and
then the planarizer removes the excess build material from each
layer to achieve the desired thickness. The use of the planarizer
is preferred generally because it does not require an active
feedback system that monitors the surface condition of a given
layer. Importantly, however, planarizing must be completed for a
given layer prior to curing the layer.
In an alternative embodiment for normalizing the layers, a surface
scanning system can be provided. Such a system would actively
monitor the surface condition of any given layer and provide
feedback data that can be used to selectively dispense additional
material in low areas to form a uniform layer. One such system is
disclosed in U.S. patent application Ser. No. 09/779,355 to
Kerekes, filed on Feb. 8, 2001 which is herein incorporated by
reference as set forth in full. Such a closed loop system would be
desirable to actively control the accumulation of material forming
the layers. Such a system could increase build speed by eliminating
the necessity to dispense material in excess of that required for a
layer which is then removed by a planarizer. Hence a surface
scanning system may be used, if desired, in conjunction with the
present invention to normalize the layers.
A waste collection system (not shown in FIG. 1) is used to collect
the excess material generated during planarizing. The waste
collection system may comprise an umbilical that delivers the
material to a waste tank or waste cartridge, if desired. A
preferred waste system for curable phase change materials is
disclosed in the concurrently filed U.S. patent application Ser.
No. 09/970,956 titled "Quantitized Feed System for Solid Freeform
Fabrication", assigned to 3D Systems Inc., which is herein
incorporated by reference as set forth in full.
In the embodiment shown in FIG. 1, a single print head dispenses
both the curable phase change material and the non-curable phase
change material. Alternatively, multiple print heads could be used,
each being dedicated to dispensing either or both of the materials.
Preferably the non-curable material is selected so as to be easily
removed from the three-dimensional object at the end of the layer
wise build process, yet have a similar melting point and freezing
point as the curable material so that dispensing and planarizing
will be uniform. In this embodiment, separate material delivery
systems are required for the two different materials, however only
one waste collection system is needed since the waste is a
combination of both materials collected after planarizing.
Unique to the SDM apparatus 10 is the provision of an actinic
radiation source generally shown by numeral 36 mounted on rail
system 18. The radiation source 36 is reciprocally driven along
rail system 18 to position the radiation source over a just formed
layer of material. The radiation source 36 includes an ultraviolet
radiation-emitting bulb 38 which is used to provide flood exposure
of UV radiation to each layer after the planarizer has normalized
the layer. Alternatively multiple layers can be dispensed and
normalized prior to curing by flood exposure to UV radiation. The
exposure is executed in a flash manner, preferably by turning on
and off the bulb 38 at a desired time, such as after the planarizer
has been retracted from the build area and while the radiation
source is traversed along the rail system over the build area.
Alternatively, the bulb could remain on and a shutter system could
be used to control the flash operation of exposure, if desired.
Although the actinic radiation source 36 is shown reciprocally
mounted on rail system 18, it may be mounted directly on the
dispensing trolley, if desired. It is important to shield the print
head and planarizer from exposure to the actinic radiation so as to
prevent curing material in the dispensing orifices or on the
surface of the planarizer, either of which would ruin the build
process and damage the apparatus.
Preferably, an external computer 34 generates or is provided with a
solid modeling CAD data file containing three-dimensional
coordinate data of an object to be formed. Typically the computer
34 converts the data of the object into surface representation
data, most commonly into the STL file format. In the preferred
embodiment, the computer also establishes data corresponding to
support regions for the object. A detailed description of
techniques for establishing the data corresponding to support
regions is provided in U.S. Pat. No. 5,943,235 which is
incorporated herein by reference. When a user desires to build an
object, a print command is executed at the external computer in
which the STL file is processed, through print client software, and
sent to the computer controller 40 of the SDM apparatus 10 as a
print job. The processed data transmitted to the computer
controller 40 can be sent by any conventional data transferable
medium desired, such as by magnetic disk tape, microelectronic
memory, network connection, or the like. The computer controller
processes the data and executes the signals that operate the
apparatus to form the object. The data transmission route and
controls of the various components of the SDM apparatus are
represented as dashed lines at 42.
The formulations for the build material and support material are
dispensed by the SDM apparatus 10 while in a flowable state. The
build and support formulations solidify substantially upon contact
with the build platform 14 for the first layer, and on top of
previously formed layers for subsequent layers. The freezing point
of the material, the point the material solidifies to the
non-flowable state, is desired to be in a range of between about
40.degree. C. to about 80.degree. C. Preferably the actual freezing
point should lean towards the higher temperature, if possible, to
assure solidification in light of exothermic heat being generated
during cure.
After all the material for each layer is dispensed and solidified,
a planarizer 32 is then used to normalize each layer. After
normalization, each layer is then provided with a flood exposure to
UV radiation by radiation source 38 which is part of an exposure
trolley 38. The flood exposure cures the build material and not the
support material. The support material is removed to expose the
three-dimensional objects.
The support material is removed by further processing. Generally,
application of thermal heat to bring the support material back to a
flowable state is needed to remove substantially all of the support
material from the three-dimensional object. This can be
accomplished in a variety of ways. For example, the part can be
placed in a heated vat of liquid material such in water or oil.
Physical agitation may also be used, such as by directing a jet of
the heated liquid material directly at the support material. This
can be accomplished by steam cleaning with appropriate equipment.
Alternatively, the support material can also be removed by
submersing the material in an appropriate liquid solvent to
dissolve the support material.
Referring particularly to FIG. 2 there is illustrated generally by
the numeral 10 a preferred embodiment of a solid freeform
fabrication apparatus for practicing the present invention. This
apparatus 10 is shown including schematically a material feed and
waste system illustrated generally by numeral 54. In contrast to
the SDM apparatus shown in FIG. 1, the build platform 14 in this
embodiment is reciprocally driven by the conventional drive means
26 instead of the dispensing trolley 20. The dispensing trolley 20
is precisely moved by actuation means 16 vertically to control the
thickness of the layers of the object. Preferably the actuation
means 16 comprises precision lead screw linear actuators driven by
servomotors. In the preferred embodiment the ends of the linear
actuators 16 reside on opposite ends of the build environment 12
and in a transverse direction to the direction of reciprocation of
the build platform. However for ease of illustration in FIG. 2 they
are shown in a two-dimensionally flat manner giving the appearance
that the linear actuators are aligned in the direction of
reciprocation of the build platform 14. Although they may be
aligned with the direction of reciprocation, it is preferred they
be situated in a transverse direction so as to optimize the use of
space within the apparatus.
In the build environment generally illustrated by numeral 12, there
is shown by the numeral 44 a three-dimensional object being formed
with integrally formed supports 46. The object 44 and supports 46
both reside in a sufficiently fixed manner on the build platform 14
so as to sustain the acceleration and deceleration effects during
reciprocation of the build platform while still being removable
from the platform. In order to achieve this, it is desirable to
dispense at least one complete layer of support material on the
build platform before dispensing the build material since the
support material is designed to be removed at the end of the build
process. In this embodiment, the curable phase change build
material identified by numeral 22 is dispensed by the apparatus 10
to form the three-dimensional object 44, and the non-curable phase
change material identified by numeral 48 is dispensed to form the
support 46. Containers identified generally by numerals 56A and 56B
respectively hold a discrete amount of these two materials 22 and
48. Umbilicals 58A and 58B respectively deliver the material to the
print head 24. The materials 22 and 48 are heated to a flowable
state, and heaters (not shown) are provided on the umbilicals 58A
and 58B to maintain the materials in the flowable state as they are
delivered to the print head 24. In this embodiment the ink jet
print head is configured to dispense both materials from a
plurality of dispensing orifices so that both materials can be
selectively dispensed in a layerwise fashion to any target location
in any layer being formed. When the print head 24 needs additional
material 22 or 48, extrusion bars 60A and 60B are respectively
engaged to extrude the material from the containers 56A and 56B,
through the umbilicals 58A and 58B, and to the print head 24.
The dispensing trolley 20 in the embodiment shown in FIG. 2
comprises a heated planarizer 32 that removes excess material from
the layers to normalize the layers being dispensed. The heated
planarizer contacts the material in a non-flowable state and
because it is heated, locally transforms some of the material to a
flowable state. Due to the forces of surface tension, this excess
flowable material adheres to the surface of the planarizer, and as
the planarizer rotates the material is brought up to the skive 62
which is in contact with the planarizer 32. The skive 62 separates
the material from the surface of the planarizer 32 and directs the
flowable material into a waste reservoir, identified generally by
numeral 64 located on the trolley 20. A heater 66 and thermistor 68
on the waste reservoir 64 operate to maintain the temperature of
the waste reservoir at a sufficient point so that the waste
material in the reservoir remains in the flowable state. The waste
reservoir is connected to a heated waste umbilical 70 for delivery
of the waste material to the waste receptacles 72A and 72B. The
waste material is allowed to flow via gravity down to the waste
receptacles 72A and 72B. Although only one umbilical 70 with a
splice connection to each waste receptacle is shown, it is
preferred to provide a separate waste umbilical 70 between the
waste reservoir 64 and each waste receptacle 72A and 72B. For each
waste receptacle 72A and 72B, there is associated a solenoid valve
74A and 74B, for regulating the delivery of waste material to the
waste receptacles. Preferably the valves 74A and 74B remain closed,
and only open when the respective extrusion bars 60A and 60B are
energized to remove additional material. For example, if only
extrusion bar 60A is energized, only valve 74A will open to allow
waste material 76 to be dispensed into the waste receptacle 72A.
This feedback control of the valves prevents delivery of too much
waste material to either waste receptacle, by equalizing the
delivery of waste material in the waste receptacles in proportion
to the rate at which material is feed from the containers to the
dispensing device. Thus, the delivery of waste material to the
waste receptacles is balanced with the feed rates of build material
and support material of the feed system.
After the curable phase change build material 22 and non-curable
phase change support material 48 are dispensed in a layer, they
transition from the flowable state to a non-flowable state. After a
layer has been normalized by the passage of the planarizer 32 over
the layer, the layer is then exposed to actinic radiation by
radiation source 78. Preferably the actinic radiation is in the
ultraviolet or infrared band of the spectrum. It is important,
however, that planarizing occurs prior to exposing a layer to the
radiation source 78. This is because the preferred planarizer can
only normalize the layers if the material in the layers can be
changed from the non-flowable to the flowable state, which cannot
occur if the material 22 is first cured.
In this embodiment, both materials accumulate and are removed by
the planarizer 32 to form the waste material. Preferably, a second
radiation source 80 is provided to expose the waste material in the
waste receptacles to radiation to cause the build material 22 in
the receptacles to cure so that there is no reactive material in
the waste receptacles.
In any SFF process, including an SDM process, estimating and then
tracking usage of the materials is an important need for a user of
the technology. An accurate prediction of the amount of both build
and support material requirements before a build begins enables a
user to better predict consumptions and therefore costs of a build.
This can help in making competitive quotes if the user is a service
provider. In addition there is a need in such a system for an
automatic notification to the user if the system currently has
sufficient materials already loaded to complete the next build
package as well as a prediction of when materials should be added
to the system. Such a system should also track material consumption
over time such as a week or month or other desired time period, to
help the user in monitoring consumption and therefore costs. It is
a vital tool for process planning and cost estimating. To do all of
these things the software needs to estimate the amount of material
used to build the model and its supports and do so quickly before
the build process begins. As different materials are used for the
models and supports, the material consumption for both the models
and its supports need to be calculated separately in the software.
The process for doing this is fairly well understood for the build
material because the description of the objects to be built are
normally in a CAD or STL data format before the build begins and
the methods for calculating the volume of such CAD or STL described
objects are known. As described in U.S. Pat. No. 5,943,235
(referenced earlier) however, the data for the support material is
often not available before the build begins, and is in fact often
generated during the build. An aspect of the instant invention that
enables the aforementioned needs to be met is a new approach to
calculating the volume and therefore the weight of the support
material before the build begins. That approach and technique is
described below.
The total material used to build objects is equal to the material
used for the actual objects and the material used to build their
supporting structure (supports.) plus any waste material generated
by the process. For example, FIG. 3 shows a horizontal cylinder,
and the dotted lines represent the supports for the horizontal
cylinder. If the cylinder is represented in CAD or STL notation the
volume and hence the weight of it can be easily calculated using a
standard formula. For example since an STL representation of an
object is a collection of triangles we know that the exact volume
of a triangulated model (V.sub.model) can be calculated based on
the sum of the signed volume of each tetrahedron formed by a
triangle and the original point O(0, 0, 0).
That is, V.sub.model =.SIGMA.V.sub.O-Ti, 1 , . . . , Num.sub.Tri.
For a triangle T.sub.i with three vertices A, B, C,
The real problem arises while calculating the volume (and then the
weight) of the supports because a digital description (CAD or STL)
is not available for the supports before the build process. An
aspect of the instant invention is a technique that has been
implemented to calculate the support material weight. This
technique will be described for STL models but can be easily
implemented for other CAD models. Whatever technique is used needs
to be rapid and accurate. The calculation time for both supports
and build material for a 15 MB STL model should be less than 15
seconds with current computer power. Support volume errors should
be less than 10%.
Note that after calculating either the build or support material
volumes and weights that a small correction factor must be added to
account for the waste materials that are removed as a result of the
planarizer action or any other known creation of waste. This factor
is a small correction and can be pre-estimated based on the
pre-defined level of waste removal.
For a given orientation of the model, all the down-facing triangles
need to be supported. A point to note is that the supports do not
always land on the platform. Instead they can land on an up-facing
triangle of itself. FIG. 3 shows an object and it's associated
supported structure generally indicated by the numeral 90 and
includes the object 92, the supports 94, and the platform 96 on
which it is built. The upper portion of the inner radius of a
horizontal cantilever cylinder is down-facing and will require
supports. These supports land inside the cylinder itself (at the
up-facing region of the inner radius) and don't go through the
cylinder. In more complicated situations, the supports can even
land on the up-facing triangles of another part (as shown in FIG.
4). FIG. 4 exhibits two objects and their supports generally
indicated generally by the numeral 100. Two horizontal cylinders
102 are supported by support structures 104 connected to a build
platform 106. The process of identifying the exact up-facing
triangles is very cumbersome and time consuming. Especially if the
supports land on only half the triangle, then the triangles needed
to be split into two for calculation purposes. Hence, the technique
of identifying up-facing triangles was not used. Instead the
approach described below was developed.
Theoretically, we can project all the triangles of a model or
models in Z-direction onto the platform to get a volume called the
sweeping body. For example, the sweeping bodies in FIGS. 5 and 6
correspond to the cases in FIGS. 3 and 4 respectively. In FIG. 5
the sweeping body concept is indicated generally by the numeral 110
in which a sweeping body 112 is indicated and the corresponding
model or object 116 is shown being subtracted out according to the
formula now presented. Assume that the volumes of the sweeping body
and the original model are V.sub.sweep and V.sub.model. Then the
volume under the sweeping body associated with the support material
is:
FIG. 6 shows the same concept for more than one object or model.
The sweeping body 122 incorporates all of the two models and the
supports and the models 124 are then subtracted out to leave the
sweeping body support volume.
Note that this concept is viable regardless of whether the support
material is solid and covers 100% of the down facing surfaces or if
one of many of the proposed support styles referenced in U.S. Pat.
No. 5,943,235 (incorporated by reference earlier) is used. The
actual volume of support material will be 100% of V.sub.support
when the support style is a complete solid and some known fraction
of V.sub.support based on other support styles. Those fractions can
be pre-calculated and stored in the system computer for each style.
Similarly the weight of support material can be calculated from the
sweeping body support volume V.sub.support by multiplying it by the
density of the supports structure which is a characteristic density
pre-calculated for a given support style and stored in a table in
the client software.
It is non-trivial however, to calculate the exact volume of the
sweeping body (V.sub.sweep), as it is time consuming to get all the
triangles that form the sweeping body. Hence the following
technique, based on the X-Y extents of the objects is used. The
extents of a CAD or STL model of an object is the Cartesian
coordinate bounding box (in x, y, and z) that can be drawn around
the object to exactly enclose it in all dimensions. Z is the
vertical dimension. The X-Y extents refers to the "top lid` of such
a bounding box. The process is as follows:
1. Map a square X-Y grid (with x-y dimensions of .sup.3 d.sup.2)
across the X-Y extents of the models.
2. For each cell in the grid, determine the topmost triangle of the
STL models of the object below.
3. Find the Z-height of that portion of the triangle that is right
under the cell.
4. Record this Z-height in this cell.
5. Repeat this process for all the cells and store 0 if there is
not a triangle under the cell.
6. Each cell projected down to the platform defines a rectangular
block.
7. Find the volume of all such rectangular blocks where the
Z-height is not 0.
8. Add all the volumes to give the approximate V.sub.sweep
9. Calculate the sweeping body support volume by subtracting the
model volume from the sweeping-body volume.
10. Multiply the sweeping body support volume by the support
structure density to get the support weight. Support structure
density is the support material density adjusted to account for the
support style. For a 100% solid support the support structure
density is the support material density. The density of supports
structure is a constant for a given support style and will be
stored in a table in the client software.
11. Add in the amount of waste support material. The waste support
and waste build materials are calculated from total waste by the
ratio of the calculated support and build materials.
It should be noted that there is a trade-off between accuracy and
computational speed based on the size of the grid. Choosing grid
size will be explained below.
The volume of the sweeping body is approximated by a Z-buffer
corresponding to the grids of the platform as shown in FIG. 7. The
x-y extent for an object or model is shown generally by the numeral
130. The y-extent 132 represents the complete y dimension scale of
the object and the x-extent 134 represents the x dimension. The
grid size 136 is a square of size (d) uniform across the complete
x-y extent. For a given square grid size (d), we can estimate
V.sub.sweep based on the Z values of the grids that are covered by
the model.
A z-height algorithm can be used to generate the values for the
cells. First, all the grids are initialized as Z=0. Then we go
through each up-facing triangles in the CTL file to update Z values
(since down-facing triangles will always be overlapped).
Note--A CTL file (also called a compressed-STL file) is a standard
file generated from an STL file. It consists of three modules; the
compressed file header, the vertices data, and the triangle date,
which is a set of triples of integers. Each integer is an index
into the array of vertices, indicating the vertices of the
triangle.
FIG. 8 demonstrates aspects of the calculation with one triangle as
an illustration. The triangle has vertices represented by the
points 142,144, and 146. For each triangle T.sub.i with vertices
V.sub.1, V.sub.2, V.sub.3 as shown in FIG. 8, the z values of the
grids that are covered by T.sub.i are calculated and updated based
on the following equations.
For a grid with center coordinate (X, Y),
Finally, the grid has Z(X, Y)=CZ.multidot.Y+DZ.
We compare the result with the z buffer. If the new value is
bigger, it updates the related value in the buffer. Otherwise it is
discarded. For the triangle, each column from the leftmost one
(related to X.sub.1) to the rightmost one (related to X.sub.2) is
calculated; and for each column, each row from the top one (related
to p.sub.1 or p'.sub.1) to the bottom one (related to p.sub.2 or
p'.sub.2) is calculated based on the above equations.
The following equations are used to get the grid position (i, j)
for a point (x, y):
Therefore, a grid (i, j) will be related to point (x, y) only if
the point is lower, and to the left side of the grid center. This
is shown in FIG. 9, represented generally by the numeral 150. The
center of a grid 152 is shown. Note that the leftmost vertex of a
triangle is actually the rightmost vertex of another triangle (e.g.
V.sub.1 related to T.sub.1 and T.sub.2 in FIG. 9). So the following
rules are used to avoid the duplication or leaking in the Z value
calculations for the grids.
X Direction: First column=i(X); Middle column=i(X)-1; Last
column=i(X)-1; Y Direction: First row=j(Y); Last row=j(Y)-1.
The grid size (d) is determined based on the following judgments.
Since the resolution of the print head in a preferred embodiment is
300 dpi, each pixel is 1/300 inch.apprxeq.3.3 mil. Assume d is N
times the pixel size. N=1 is the minimum grid size we should use.
If the grid size is equal to this size, the maximum memory size of
the Z-buffer is 3000.times.3000.apprxeq.9 MB, based on a platform
size of 10 inch.times.10 inch. The actual memory size for a model
depends on the X-Y extent of the model. With a bigger N,
computational time is reduced and the accuracy of the resulted
volume decreases. To examine this factor we assembled eight STL
models of increasing complexity and size, ranging from a simple
cylinder represented by 112 triangles to a complex model of an
automobile requiring over 150,000 triangles. Sweeping body
calculations were done on each and the computational time and %
error in the sweeping body volume calculations were recorded.
Results were obtained from N=1 to N=16 [d=3.3 mils to d=52.8 mils].
The average percentage volume error stayed below 1% for N up to 8
and jumped to over 3% error at N=16. A value of N=4 [13.3 mils]
seemed to be a good trade-off, giving average % volume error of
about 0.5% and computational times of less than 1 second for even
the biggest and most complex model. All tests were performed on a
Dell computer with a 1.7 GHz Intel Xeo processor and 2 GB DRAM.
Once a fast and accurate technique is available for estimating
support material volumes and weights other important estimates can
be done for the user. The estimates from one or more previous
builds can be tabulated and recorded by the system computer to
maintain a record of the material remaining in the feed cartridges.
Or in the preferred embodiment SDM process shown in FIG. 2 the
extension of the extrusion bar 60B could be tracked by the system
computer to know the volume and weight of support material that
remains in that feed cartridge (56B). Other techniques could be
used to track the amount of materials remaining in the feed
cartridges. Thus it is straightforward to use that information and
the support material estimate from this instant invention to
communicate to the user through the system computer whether the
build she is about to begin has enough support material available
to complete the build. It should be noted that the same calculation
and communication can be made to the user regarding build material
as build material requirement can be calculated from standard CAD
or STL data of the object as discussed before. The amount of build
material used by the system can be tracked by the extension of the
extrusion bar 60A (FIG. 2) in a similar manner and the user can be
notified when a build material cartridge 56A should be replaced. As
all of these computations can be tabulated and stored over time the
system can also provide reports to the user regarding consumption
of build and support materials over extended builds.
What have been described above are preferred embodiments in which
modifications and changes may be made without departing from the
spirit and scope of the accompanying claims. Accordingly, it is
intended to embrace all such changes modifications and variation
that fall within the spirit and broad scope of the appended claims.
All patent applications, patents and other publications cited
herein are incorporated by reference in their entirety.
* * * * *